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  • Prove the following (sin5x + sin3x) / (cos5x+ cos3x) = tan4x

Q (17) Prove the following

\small \frac{\sin5x + \sin3x}{\cos5x + \cos3x} = \tan4x

Answers (1)

We know that

\\ \sin A + \sin B = 2\sin\frac{A+B}{2}\cos\frac{A-B}{2}\\and\\ \\ \cos A + \cos B = 2\cos\frac{A+B}{2}\cos\frac{A-B}{2} \\

We use these identities

\\ \sin5x + \sin3x = 2\sin4x\cos x\\ \cos5 x + \cos 3x = 2\cos4x\cos x \\ \\ \frac{\sin5x + \sin3x}{\cos5 x + \cos 3x} = \frac{ 2\sin4x\cos x}{2\cos4x\cos x} = \frac{\sin4x}{\cos 4x} = \tan 4x
                                                                                                          R.H.S.

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Safeer PP

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